This invention relates generally to isolation networks, and more particularly to networks for isolating a balanced antenna system from a line connected chassis.
Modern radio and television receivers often have chassis that are connected directly to one side of the AC power line. Such receivers generally employ a polarized power plug to assure that the chassis is connected to the grounded side of the power line. However, if an unpolarized plug, an extension cord or an adapter plug is used, the power plug may be reversed. Under such conditions, the chassis will be maintained at a potential equal to the potential of the AC power line.
The radio frequency input circuits of modern receivers generally employ at least one input inductor. Such input inductors are generally grounded to the chassis to reduce radio frequency interference and the possibility of oscillation. When such an input inductor is attached to an antenna terminal, and the AC power plug is reversed, the antenna terminal will have the full AC power line potential applied to it, thereby resulting in a significant shock hazard to the operator of the receiver.
A known technique for isolating an antenna terminal from a line connected chassis utilizes a capacitor connected in a series circuit between the receiver input inductor and the antenna terminal. The value of the capacitor is chosen such that the capacitor provides a relatively low impedance to radio frequency signals and a relatively high impedance to the sixty cycle power line frequency, thereby effectively isolating the antenna terminal from the line connected chassis. Because a static charge of sufficient magnitude to break down the isolating capacitor can build up on the capacitor during an electrical storm or as a result of an electrically charged cloud, a bleeding resistor is generally connected in parallel with the isolating capacitor to discharge any built up static charge. The value of the bleeding resistor is chose to be low enough to effectively discharge the capacitor, and high enough to limit the flow of AC line current between the chassis and the antenna terminal to a safe value. A resistor having a resistance of approximately 5 megohms has been found satisfactory for such purposes. If the resistance is increased significantly above 5 megohms, the capacitor may not be properly discharged, whereas if the value of the resistor is reduced substantially below 5 megohms, not enough isolation is provided to avoid the possibility of a shock hazard.
When a single antenna input terminal is used, adequate isolation can be provided by utilizing a capacitor and a resistor having a resistance in the range of 3 to 5 megohms. When a balanced antenna system having two input terminals is utilized, each terminal must be isolated by means of a separate isolating capacitor and resistor connected in series with each input terminal. The results in two separate paths between the antenna terminals and the chassis. As a result, if both antenna terminals, or both arms of a dipole antenna are simultaneously grasped, the effective resistance between the antenna or terminals and the chassis is equal to the parallel combination of the two isolating resistors. Because neither of the resistors can be reduced to a value of less than approximately 5 megohms and still prevent excessive static charge buildup, the effective resistance between the two antenna terminals and the chassis will be approximately 2.5 megohms. The results in marginal isolation for safety purposes between the antenna terminals and the line connected chassis.